In a vehicle equipped with electric power steering (EPS), a steering assist force is provided by an electric motor operatively coupled to a steering wheel through a steering column or shaft. Typically, such systems include a controller programmed to derive an assist torque signal along with a “return to center” (i.e., neutral position) torque signal, thereafter summing these torque signals to generate a motor command signal. The assist torque signal is derived using an applied torque signal indicative of torque applied to the steering wheel by a vehicle operator. The assist torque signal provides a power steering boost torque, and the neutral position torque signal provides a return-to-center bias torque.
During EPS operation, the motor command signal is applied to the electric motor, and a sensing device is utilized to sense the angular velocity of the electric motor. Based upon the angular velocity of the electric motor and the applied torque signal, four operational quadrants may be defined. Quadrant I is defined as a motor operating condition wherein the applied torque signal and the angular velocity are both positive. Quadrant II is defined as a motor operating condition wherein the applied torque signal is negative but the angular velocity is positive. In practice, Quadrant II situations arise when the applied torque signal specifies a reversal in motor rotation. In Quadrant III, the applied torque signal and the angular velocity are both negative, whereas in Quadrant IV, the applied torque signal is positive, but the angular velocity is negative. Quadrant III mirrors Quadrant I, but with opposite signs for angular velocity and applied torque signals. Similarly, Quadrant IV mirrors Quadrant II, but with opposite signs for angular velocity and applied torque signals. Although Quadrants I and II are discussed herein, it should be noted that any discussion of Quadrant I also applies to Quadrant III, and also that any discussion of Quadrant II also applies to Quadrant IV.
As a practical matter, EPS systems may exhibit free rotational oscillation resonances that, if left undamped, may not result in a crisp, controlled feeling to the steering. EPS systems are non-linear, providing a steering assist force which varies as a function of vehicle speed, road conditions, and the driving maneuvers being performed. In spite of these variations, good EPS system performance is characterized by linear behavior as perceived from the driver's point of view. Drivers expect steering behavior to remain consistent throughout a wide variety of operating conditions.
In order to impart a stable and precise feel to an EPS system, input-dependent active damping may be utilized. An example of such a technique is presented in U.S. Pat. No. 5,919,241 (“the '241 patent”), Vehicle Having Electric Power Steering With Active Damping, filed Dec. 13, 1996, incorporated by reference herein in its entirety and assigned to the assignee of the present application. In the '241 patent, active damping provides an active damping torque signal that is further summed along with the assist torque signal and the return to center torque signal to produce the torque command signal. This active damping torque signal is derived as a function of a filtered steering shaft position and a sensed vehicle velocity. A filtering mechanism for generating the steering shaft angular velocity applies the amplitude and phase characteristics of a differentiator from 0 Hz through the resonant frequency of free rotational oscillation of the EPS system to the steering shaft position.
A continuing source of difficulty in tuning and controlling EPS systems is a perception by the driver of a difference in damping between operation in Quadrant I as opposed to Quadrant II. However, existing active damping techniques do not modify damping behavior in response to a quadrant transition. As a result, these techniques provide undesirable, inappropriate, and oftentimes annoying tactile feedback to the steering wheel throughout one or more quadrants. Accordingly, what is needed is an improved technique for applying active damping to an EPS system that may operate in any of a plurality of quadrants.